The MIMO system is a communication system that uses multiple transmission antennas and multiple reception antennas for data communication. In a multiple-access MIMO system in which a plurality of wireless communication apparatuses are simultaneously accessible, an access point to which a user equipment is connected may communicate with one or more user equipments at a certain time point. In the case where the access point communicates with a single user equipment, a plurality of transmission antennas are related to one transmission substance (any one of an access point and a user equipment), and a plurality of reception antennas are related to one reception substance (any one of the user equipment and the access point).
This access point can also communicate with a plurality of user equipments through communication of SDMA (Space Division Multiple Access). Hereinafter, the multiple-access MIMO system using the SDMA is called a MU-MIMO (Multi User MIMO) system. In an MU-MIMO system in the related art, an access point uses a plurality of antennas for data transmission and reception, and each user equipment typically uses one antenna for data transmission and a plurality of antennas for data reception. In a downlink and an uplink, the access point can communicate with one or more user equipments at a certain time point. The downlink (that is, forward link) is a communication link from the access point to the user equipment, and the uplink (that is, backward link) is a communication link from the user equipment to the access point.
The access point is a wireless communication apparatus achieved by a fixed base station that typically communicates with a user equipment, and may be called a base station or another technical term. The user equipment is a fixed or movable wireless communication apparatus, and may be called a base station, a wireless device, a mobile station, a user device, or another technical term. Hereinafter, it is assumed that a BS (Base Station) is used for an access point and a UE (User Equipment) is used for a user equipment.
Several important subjects in an MU-MIMO system for an uplink are (1) selection of a UE group that is appropriate for simultaneous communication and (2) respective UEs selected by a method for achieving good system performance through the reduction of interference and/or data transmission between the respective UEs. Precoding that uses a limited feedback signal of channel information for each UE has been proposed to improve the performance of the system that uses the SDMA (for example, see Patent Literature 1 below). In the MIMO system, precoding is a technique for performing transmission by forming beams that are appropriate to the situation of a propagation path through transmission of weighted data from a plurality of antennas when the transmission is performed from the plurality of antennas. In this case, in order to reflect the observed situation (propagation path situation) of a received signal in a reception point, a feedback signal that includes beam information is transmitted from a receiver to a transmitter, and the transmitter controls the beams using the feedback signal.